As is well known, internal combustion engines generate vibrations during operation. These vibrations get transmitted to the vehicle or device to which they are mounted. Engine mounts are typically mounted between the engine and the vehicle or device to actively or passively reduce the transmission of the vibrations thereto. The effectiveness of the engine mounts is related to both their type and their location amongst other factors. Engine mounts will also typically be more effective over certain ranges of speed of the engine.
FIG. 1 schematically illustrates a top view of a typical engine mount system used in marine outboard engines. An engine 1 has a crankcase 3 and one or more cylinders 5 extending horizontally away from a boat (not shown) to which the marine outboard engine is mounted. A piston 7 is disposed in each cylinder 5. Each piston 7 is pivotally connected by a wristpin 9 to a connecting rod 11. Each connecting rod 11 connects its respective piston to a crankshaft 13 of the engine 1. The engine 1 is connected to a bracket 15 that is pivotally connected to a steering shaft 17 about which the outboard engine is pivoted to be steered. A tiller 19 extends from the bracket 15 to allow a user of the outboard marine engine to manually steer the outboard marine engine. Alternatively, the bracket 15 could be connected to a steering mechanism such as the steering wheel of a boat. A stern bracket (not shown) is pivotally connected to the steering shaft 17 and pivotally connects the marine outboard engine to the transom of the boat. Two or more engine mounts 21 are connected between the engine 1 and the bracket 15 to reduce the transmission of vibrations from the engine 1 to the tiller 19. The working axes 23 of the engine mounts 21 (i.e. the axes along which the engine mounts 21 absorb the vibrations) are arranged parallel to the cylinder axis 25.
The engine mounts 21 are arranged this way since at high engine speeds the engine 1 vibrates primarily in a fore and aft direction generally along the cylinder axis 25 (in an up down direction in FIG. 1). Thus having the working axis 23 of the engine mounts 21 arranged parallel to the direction of the vibration provides adequate damping for such engine operating speeds.
At low engine speeds however, the primary source of engine vibrations for an in-line engine 1 such as the one illustrated in FIG. 1 is what is known as torque-kick. Torque-kick is the reaction of the engine block (crankcase 3 and cylinder 5) to the force F on the wall of the cylinder 5 adjacent to the wrist pin 9 during combustion. This side force F is the result of the connecting rod 11 forming an angle with respect to the cylinder axis 25 while the piston 7 is loaded by combustion pressure in the direction of the cylinder axis 25. The torque-kick creates an alternating moment about the torque-roll axis 27 of the engine 1. This moment causes the engine 1 rotate/vibrate about the torque-roll axis 27. Therefore, by having the working axes 23 of the engine mounts 21 arranged as shown, the force reactions at the engine mounts 21 to the moment generated at low engine speeds are applied to a moment arm having a length D and create a moment M about the steering axis 29 of the steering shaft 17. This moment M generated about the steering axis 29 is then transmitted to the tiller 19 as vibrations.
Thus, although the engine mount system illustrated in FIG. 1 provides adequate vibration damping at high engine speeds, it provides less effective vibration damping at lower engine speeds where the primary source of engine vibrations is torque-kick.
Therefore, there is a need for an engine mount system for a marine outboard engine that better dampens vibrations due to torque-kick.
There is also a need for an engine mount system for a marine outboard engine that better dampens vibrations over a broad range of engine speeds.